British Pat. No. 1,291,504 to Webster discloses an instrument in which first and second mirrors are used to produce an image of a second object which substantially coincides with an image of a first object viewed over the first mirror. However, this instrument is not easy to use, and is not very accurate. Although there is a zero line marked on the instrument, the observer has to move his head one side of the zero line to see an image of a second object, and the other side of the zero line to see an image of a third object. At the same time, the images will not be entirely coincident because of this small deflection of the returning rays from the centre line of the instrument.